1. Technical Field of the Invention
This invention relates generally to communication systems and more particularly to high data rate serial communications.
2. Description of Related Art
Communication systems are known to transport large amounts of data between a plurality of end user devices, which, for example, include telephones (i.e., land line and cellular), facsimile machines, computers, television sets, personal digital assistants, etc. As is known, such communication systems may be local area networks (LANs) and/or wide area networks (WANs) that are stand-alone communication systems or interconnected to other LANs and/or WANs as part of a public switched telephone network (PSTN), packet switched data network (PSDN), integrated service digital network (ISDN), or the Internet. As is further known, communication systems include a plurality of system equipment to facilitate the transporting of data. Such system equipment includes, but is not limited to, routers, switches, bridges, gateways, protocol converters, frame relays, and private branch exchanges.
The transportation of data within communication systems is governed by one or more standards that ensure the integrity of data conveyances and fairness of access for data conveyances. For example, there are a variety of Ethernet standards that govern serial transmissions within a communication system at data rates of 10 megabits per second, 100 megabits per second, 1 gigabit per second (Gbps) and beyond. Synchronous Optical NETwork (SONET), for example, currently provides for up to 10 Gbps. In accordance with such standards, many system components and end user devices of a communication system transport data via serial transmission paths. Internally, however, the system components and end user devices may process data in a parallel manner. As such, each system component and end user device must receive the serial data and convert the serial data into parallel data without loss of information. After processing the data, the parallel data must be converted back to serial data for transmission without loss of information.
Accurate recovery of information from high-speed serial transmissions typically requires transceiver components that operate at clock speeds equal to or higher than the received serial data rate. Higher clock speeds require oscillator circuits to have gain-bandwidth products to sustain high frequency oscillations while maintaining low phase noise. High phase noise contributes to clock jitter which degrades clock recovery in high-speed circuits. Higher clock speeds limit the usefulness of prior art clock recovery circuits that require precise alignment of signals to recover clock and/or data. Higher data rates require greater bandwidth for a feedback loop of the clock recovery circuits to operate correctly. Some prior art designs are bandwidth limited.
As the demand for data throughput increases, so do the demands on a high-speed serial transceiver. The increased throughput demands are pushing some current integrated circuit manufacturing processes to their operating limits. Integrated circuit processing limits (e.g., device parasitics, trace sizes, propagation delays, device sizes) and integrated circuit (IC) fabrication limits (e.g., IC layout, frequency response of the packaging, frequency response of bonding wires) limit the speed at which the high-speed serial transceiver may operate without excessive jitter performance and/or noise performance.
One solution for high-speed serial transceivers is to use an IC technology that inherently provides for greater speeds. For instance, switching from a Complementary Metal-Oxide Semiconductor (CMOS) process to a silicon germanium or gallium arsenide process would allow integrated circuit transceivers to operate at greater speeds, but at substantially increased manufacturing costs. CMOS is more cost effective and provides easier system integration. Currently, for most commercial-grade applications, including communication systems, such alternate integrated circuit fabrication processes are too cost prohibitive for widespread use.
Modern communication systems, including high data rate communication systems, typically include a plurality of circuit boards that communicate with each other by way of signal traces, bundled data lines, back planes, etc. Accordingly, designers of high data rate communication transceiver devices often have conflicting design goals that relate to the performance of the particular device. For example, there are many different communication protocols specified for data rates that range from 2.48832 Gbps for OC48, to 9.95 Gbps for OC192. Other known standards define data rates of 2.5 Gbps (INFINIBAND) or 3.125 Gbps (XAUI). For example, one protocol may specify a peak voltage range of 200-400 millivolts, while another standard specifies a mutually exclusive voltage range of 500-700 millivolts. Thus, a designer either cannot satisfy these mutually exclusive requirements (and therefore cannot support multiple protocols) or must design a high data rate transceiver device that can adapt according to the protocol being used for the communications.
Along these lines, field programmable gate array (FPGA) circuits are gaining in popularity for providing the required flexibility and adaptable performance, as described above, for those designers that seek to build one device that can operate according to multiple protocols. Thus, while FPGA technology affords a designer an opportunity to develop flexible and configurable hardware circuits, specific designs that achieve the desired operations must still be developed.
High data rate serial communications, whether produced or processed by FPGA circuits, or whether processed by traditional Application Specific Integrated Circuits (ASIC) or other types of communication devices, often experience signal distortion in the communication channel. Generally, higher frequency signals or data rates result in greater amounts of added distortion. One known distortion relates to introduced phase shifts in the serial data. Additionally, a square wave often experiences attenuation of higher frequency components of the square wave resulting in a signal that appears as a sine wave. Thus, for high data rate serial communications, the signal magnitude is often significantly attenuated thereby rendering the signal more difficult to properly interpret or detect. For these and other reasons, it is desirable to equalize the signal to improve the signal quality received by the receiver. While it is generally known that equalization of a signal improves signal quality at the receiver, the circuitry and method for determining how much equalization is required is not as definite. In some prior art designs, a standard amount of equalization is performed without consideration of actual channel conditions. For those circuits and methods that might attempt, however, to measure channel conditions or channel-induced distortion, it is required that accurate measurements of the received signal magnitude be obtained to determine how much equalization is to be performed or pre-emphasis is to be added. What is needed, therefore, is a system and method for accurately measuring and determining an amount of a signal magnitude for a received signal as a part of determining how much equalization is to be performed or pre-emphasis is to be added to a signal at the transmitter end.
Generally, equalization of frequency-dependent magnitude distortion is necessary for communication over copper cable or copper circuit board traces at gigabit data rates. The amount of equalization, however, is a function of cable and trace length. Communication channels quite often are found in pairs for carrying bi-directional non-return to zero (NRZ) data. The lengths of cable or circuit board traces are typically the same for a given pair. To determine an amount of transmit-based equalization (pre-emphasis) or receiver based equalization that is to be added, it is important to accurately determine the amount of distortion or signal degradation (attenuation) that occurs in the transmission channels. Accordingly, a system and method for accurately determining signal magnitude attenuation is necessary in order to determine a proper amount of transmit-based equalization.
Data communication schemes at rates in access of 1 Gbps tend to use very high frequencies, often in access of 1 GHz. Transmission lines made with copper cable or copper circuit board traces have a frequency response as magnitude drops with increasing frequency. If the signaling method or line code encompasses a broad range of frequencies, the lower frequency components are transmitted with no attenuation, while higher frequency components can be severely attenuated. This distortion, often known as inter symbol interference, makes the detection of the signal at the receiver difficult if not impossible. Generally, the amount of attenuation, at a given frequency, is a function of the length of the cable or circuit board trace.
Many forms of equalization exist for the purpose of correcting or partially correcting the signal distortion due to frequency-dependent losses. Each of the forms of equalization effectively remove or decrease the frequency-dependence by increasing the amplitude of the higher frequency components or decreasing the amplitude of the lower frequency components. This is done in a way such that the frequency response is an inverse of transmission line frequency response characteristics. A preferred equalization method often includes use of pre-emphasis.
Pre-emphasis involves boosting one or more of the higher frequency components in the transmitter. The amount of correction that is applied is a function of the transmission line length. Since the length is not usually known, however, it is helpful to make the equalization adaptive. Accordingly, the transmitter is required to determine, to optimize the amount of pre-emphasis that is added, how much attenuation exists for higher frequency components and thus how much pre-emphasis to apply. Accordingly, what is needed is a system and method that accurately measures signal magnitudes as a part of determining how much equalization is to be performed or how much pre-emphasis is to be added to a signal at a transmitter end. Moreover, what is needed is a system and method for adaptively adding equalization or pre-emphasis based upon circuits in which line or trace links are approximately equal in the transmit and receive directions and in circuits where the transmit and receive line or trace links are unequal, thereby requiring differing amounts of equalization.